content: /* Copyright (C) 2016-2018 Alibaba Group Holding Limited
  
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at
  
      http://www.apache.org/licenses/LICENSE-2.0
  
  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.
  ==============================================================================*/
  
  // Protocol Buffers - Google's data interchange format
  // Copyright 2008 Google Inc.  All rights reserved.
  // https://developers.google.com/protocol-buffers/
  //
  // Redistribution and use in source and binary forms, with or without
  // modification, are permitted provided that the following conditions are
  // met:
  //
  //     * Redistributions of source code must retain the above copyright
  // notice, this list of conditions and the following disclaimer.
  //     * Redistributions in binary form must reproduce the above
  // copyright notice, this list of conditions and the following disclaimer
  // in the documentation and/or other materials provided with the
  // distribution.
  //     * Neither the name of Google Inc. nor the names of its
  // contributors may be used to endorse or promote products derived from
  // this software without specific prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  package com.google.protobuf;
  
  /**
   * Grab-bag of utility functions useful when dealing with RPCs.
   *
   * @author kenton@google.com Kenton Varda
   */
  public final class RpcUtil {
    private RpcUtil() {}
  
    /**
     * Take an {@code RpcCallback<Message>} and convert it to an
     * {@code RpcCallback} accepting a specific message type.  This is always
     * type-safe (parameter type contravariance).
     */
    @SuppressWarnings("unchecked")
    public static <Type extends Message> RpcCallback<Type>
    specializeCallback(final RpcCallback<Message> originalCallback) {
      return (RpcCallback<Type>)originalCallback;
      // The above cast works, but only due to technical details of the Java
      // implementation.  A more theoretically correct -- but less efficient --
      // implementation would be as follows:
      //   return new RpcCallback<Type>() {
      //     public void run(Type parameter) {
      //       originalCallback.run(parameter);
      //     }
      //   };
    }
  
    /**
     * Take an {@code RpcCallback} accepting a specific message type and convert
     * it to an {@code RpcCallback<Message>}.  The generalized callback will
     * accept any message object which has the same descriptor, and will convert
     * it to the correct class before calling the original callback.  However,
     * if the generalized callback is given a message with a different descriptor,
     * an exception will be thrown.
     */
    public static <Type extends Message>
    RpcCallback<Message> generalizeCallback(
        final RpcCallback<Type> originalCallback,
        final Class<Type> originalClass,
        final Type defaultInstance) {
      return new RpcCallback<Message>() {
        @Override
        public void run(final Message parameter) {
          Type typedParameter;
          try {
            typedParameter = originalClass.cast(parameter);
          } catch (ClassCastException ignored) {
            typedParameter = copyAsType(defaultInstance, parameter);
          }
          originalCallback.run(typedParameter);
        }
      };
    }
  
    /**
     * Creates a new message of type "Type" which is a copy of "source".  "source"
     * must have the same descriptor but may be a different class (e.g.
     * DynamicMessage).
     */
    @SuppressWarnings("unchecked")
    private static <Type extends Message> Type copyAsType(
        final Type typeDefaultInstance, final Message source) {
      return (Type) typeDefaultInstance
          .newBuilderForType().mergeFrom(source).build();
    }
  
    /**
     * Creates a callback which can only be called once.  This may be useful for
     * security, when passing a callback to untrusted code:  most callbacks do
     * not expect to be called more than once, so doing so may expose bugs if it
     * is not prevented.
     */
    public static <ParameterType>
      RpcCallback<ParameterType> newOneTimeCallback(
        final RpcCallback<ParameterType> originalCallback) {
      return new RpcCallback<ParameterType>() {
        private boolean alreadyCalled = false;
  
        @Override
        public void run(final ParameterType parameter) {
          synchronized (this) {
            if (alreadyCalled) {
              throw new AlreadyCalledException();
            }
            alreadyCalled = true;
          }
  
          originalCallback.run(parameter);
        }
      };
    }
  
    /**
     * Exception thrown when a one-time callback is called more than once.
     */
    public static final class AlreadyCalledException extends RuntimeException {
      private static final long serialVersionUID = 5469741279507848266L;
  
      public AlreadyCalledException() {
        super("This RpcCallback was already called and cannot be called " +
              "multiple times.");
      }
    }
  }
  